As the size of LCD keeps on increasing and design of the control circuit becomes complicated relatively, requirements and functions of back-lighted power supply are getting demanding. Due to an extreme increase on the number of back-lighted lamps resulted from large-sized LCDs, plenty of current problems of multiple lamps are derived, the issue of open circuit resulted from a malfunctioned lamp and protection method in particular.
The control circuit of a fluorescent lamp used for LCD back-lighted earlier had a small quantity with simple functions; therefore, design of the protection circuit was not emphasized, which was only in compliance with designated specifications. However, along with tremendous demands from the consumer market, requirements for functions and specifications have been increasing and improving. Thus, protection design tends to become more strict and complicated. At present, as LCD becomes large-sized and is used for television design at home, protection requirements are even more strict than before, which also increases complex on circuit design.
Generally speaking, the backlit driven protection circuit occupies most of the elements on the LCD and high quality of large-scale mass production is not easy. Besides, the major purpose of protection is to solve the high-voltage issue caused by an open circuit as well as to simulate a short circuit when the human body gets an electric shock. Consequently, protection function is typically designed to solve the abnormal problems caused by voltage and current of the lamp.
A general back-lighted driver circuit protection device for LCD is shown as FIG. 1, which includes circuits 16 and 17 of detecting current of the lamp and circuits 18 and 19 of detecting voltage of the lamp. The step-down loop of the voltage detection device 20 has two voltage divided impedances to reduce the high output of driven voltage from the transformer to standard signals that can be detected by general low voltage elements. This method can determine if the current potential is normally operated or if open circuit or short circuit of the lamp happens through the voltage detected. The current detection device 21 consists of current induced elements series connected to the path on which current of the LCD back-lighted lamp passes and is used to detect the status quo of the current of the lamp. Thus, condition of lamp current can be controlled through the current detection device.
An open circuit or any abnormality of the lamp can be decided via the voltage detection circuits 18 and 19 and the voltage detection device 20 or via the current detection circuits 16 and 17 and the circuit loop of the current detection device 21. Consequently, the protection circuit device 22 may be activated to protect the driver circuit of the backlit lamp 23 and prevent the transformer 24 from output of voltage.
Please refer to FIGS. 1 and 2. An example of the protection device for single lamp is given in FIG. 1 and that for dual lamps is given in FIG. 2. For the circuit design and a well protected system commonly used at present, a lot of voltage detection elements, current detection elements and circuit protection elements are required to ensure a safer operation of the high voltage from the transformer and the back-lighted lamp used. For the operation of parallel connected multiple lamps in particular, the number of elements consumed is doubled.